1. IBM Research Division, Almaden Research Center, San Jose, CA 95120, USA
2. Present address: Balzers AG, FL-9496 Balzers, Fürstentum Liechtenstein, Belgium
3. Institut für Festkörperphysik, TH-Darmstadt, W-6100 Darmstadt, Germany
4. II. Physikalisches Institut, RWTH Aachen, W-5100 Aachen, Germany